The present invention relates to a lens system of an inner focus type that can be focused at short range. Furthermore, the present invention relates to an image pickup device equipped with a lens system of an inner focus type that can be focused at short range.
Conventionally, a lens system of an inner focus type capable of being focused at short range by disposing a plurality of focus-lens groups inside the lens system has been known in, for example, JP-A-3-278012, JP-A-4-110811 and JP-A-2003-329924. Such a configuration realizes a lens system that makes it difficult for dust to get into the lens system and for a lens to collide with an object even during a focusing operation.
Meanwhile, in recent years, instead of a conventional focus detection method of a phase difference method, a focusing type has been increasingly used; according to the focusing type, a change in contrast at a time when a lens group is transferred is read, and a focus state comes when the contrast approaches a maximum. Such a focusing type makes it unnecessary to dispose a reflection surface between a lens system and an image pickup plane. Therefore, the focusing type is advantageous to making the entire device smaller when the lens system is incorporated.
Moreover, in recent years, the following lens system has been also known: the lens system with a higher auto-focusing speed, which is so formed as to cause some lenses of the lens system to slightly vibrate, or wobble, in an optical-axis direction and determine a movement direction of a focusing lens group on the basis of a change in contrast on an image pickup plane.